This invention relates to an apparatus for the spray drying of solvent-containing compositions, to a process for the spray drying of solvent-containing compositions and to uses of the apparatus according to the invention or the process according to the invention, more particularly for the production of powder-form detergents.
Apparatus for the spray drying of solvent-containing compositions, more particularly water-containing compositions, are known from the prior art.
Widely used apparatus include, for example, spray drying towers with spray nozzles which may be used in particular for converting liquid educts (solutions, suspensions or melts) into a powder-form product. In this case, the liquid is generally sprayed by pressure nozzles and then dries in a hot gas flowing in co-current or counter-current. The dry product is then removed through cyclones or filters. If a melt is sprayed and solidified in a cold gas, the tower used is known as a prilling tower.
Other known spray dryers are disk towers. Like nozzle towers, these are also short-time dryers. They use rotating disks for atomization and are compact compared with nozzle towers. The advantage of atomizing disks is that they are not affected by xe2x80x9cnozzlexe2x80x9d blockages or by highly variable liquid throughputs.
Spray dryers with an integrated fluidized bed are also known. Through the installation of a fluidized bed at the bottom of the spray drying tower, the product can be dried and graded there. The drying gas containing the fine dust is removed in the upper part of the tower at the head of the tower and, after separation, the fine dust is returned to the tower. Accordingly, even comparatively tacky and slow-drying educts can be processed. The end product is obtained in the form of readily dispersible agglomerates which are larger and hence generally lower in dust than the powders of the nozzle and disk towers.
Spray dryers in the broader sense also include fluidized bed spray granulators (xe2x80x9cagglomeration dryersxe2x80x9d) which are used for the production of granules between 0.5 mm and several mm in size from sprayable solutions, suspensions and melts. Two-component nozzles are used for spraying. The product is generally abrasion-resistant and has a relatively high bulk density. Its dissolving rate is therefore lower than that of other spray dried products. Agglomeration granulators may also be used for coating granules in which case they are generally operated discontinuously.
A key application of spray dryers is the drying of water-based preparations of wetting agents, detergents and cleaners for the production of corresponding products in pourable, free-flowing form. In nozzle or disk spray drying towers operated on an industrial scale, the products are generally obtained at ambient pressure on the co-current or more often the counter-current principle. From the extensive specialist literature available, reference is made purely by way of example to K. Masters xe2x80x9cSpray Drying Handbookxe2x80x9d, Longman Scientific and Technical 1991, ISBN 0-582-06266-7 and to the literature references cited at the internet address xe2x80x9chttp://home.t-online.de/home/michael.schoenherr/trocknungxe2x80x945.htmxe2x80x9d.
A process for the spray drying of useful materials and mixtures thereof using superheated steam is known from WO 92/05849. The use of superheated steam avoids oxidative damage to the material to be dried.
WO 96/04973 describes a process for the spray drying of water-containing useful material preparations from the field of wetting agents, detergents and cleaners which avoids caking on the inner wall of the dryer through the introduction of a powder-form auxiliary into the interior of the spray drying zone.
U.S. Pat. No. 5,723,433 describes a process for the removal of solvents from a paste-form surfactant compound which comprises introducing the paste-form compound under pressure into a drying duct and reducing the pressure at the duct entrance to such an extent that certain components of the paste-form compound undergo flash evaporation. The paste-form compound is heated in the drying duct and is driven through the duct by the gases released during flash evaporation. The end product of the process is a concentrated surfactant paste which is obtained at the outlet end of the drying duct.
Known processes for the spray drying of liquid or paste-form compositions are unfavorably characterized by a very high energy consumption. The energy input is largely used not for solvent evaporation, but rather to cover heat losses attributable, for example, to the warm waste gas.
Another disadvantage is that, in the spray drying of compositions susceptible to microbial contamination, the microorganisms present are not reliably destroyed because the compositions are generally only briefly heated and often only to a temperature below 100xc2x0 C. in the mixing vessel of the spray dryer in order to preserve the useful materials present. Even if a very hot gas stream is used for drying, the particles are only exposed to the hot gas for seconds which is not sufficient to destroy many unwanted microorganisms. This microbial challenge is particularly problematical in the production of powders or granules which are intended for use in foods, cosmetics or pharmaceuticals. Another complicating factor is that, although many microorganisms do not proliferate in the dried product in the absence of water, they do not die off either, but form extremely resistant permanent forms, such as spores for example.
Accordingly, the dried product cannot be optically or olfactorily recognized as being contaminated. However, if it is re-contacted with water, for example dissolved in water, considerable microbial contamination can occur if the product is not used within a few hours.
Accordingly, the problem addressed by the present invention was to provide a spray dryer which would avoid the above-mentioned disadvantages of the prior art.